Irazu

Costa Rica

9.979°N, 83.852°W; summit elev. 3432 m

All times are local (unless otherwise noted)

Crater fumarolic activity continued in September, with the most intense emissions in the N and NW sections, where distinct gas vents were visible. Gas temperatures were 90°C and condensates had a pH of 3.5. One fumarole produced a roaring sound audible at the summit rim overlook. The level of the turquoise-green crater lake continued to rise; numerous fumaroles were drowned but continued emitting subaqueously. The average lake temperature was 37°C (compared to 35°C in July), with a temperature of 90°C measured near active fumaroles in the N part of the lake. Weaker fumarolic activity occurred in the SE, E, and NE parts of the lake, and on the crater's E slope. In September, 74 earthquakes (M 1-2) were recorded, with depths <8 km.

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.